Many ordnance devices such as land mines are equipped with end-of-life timers which serve to detonate the device after a predetermined time interval if some other means of detonation has not occurred. Typically, these timers are set to detonate from hours to days after the mine has been laid. If this feature were not present, it would be necessary to recover and disarm all mines laid or else take the chance that at a future time innocent personnel would be harmed by an inadvertent detonation.
In the past, mechanical timers were employed to perform this function. These timers were not sufficiently accurate or reliable for all deployment situations. They were large and cumbersome and occupied an undue amount of space.
Electronic timing circuits are well known which employ resistors and capacitors to generate delayed signals. Normally, resistor-capacitor time delay circuits are employed when short delays are required. A resistor-capacitor time delay circuit with a delay time of one month would require a capacitor so large that it would be bigger than the rest of the land mine.
Many digital circuits exist for generating delayed signals. Most of these employ binary cells arranged as counters. A particular time delay is achieved by decoding a particular count for providing the time delay signal. These circuits are not the most suitable type for manufacture by integrated circuit techniques.
Therefore, it is an object of this invention to provide an improved digital timer for ordnance applications requiring extremely long time delays.
It is another object of this invention to provide a digital timer which is particularly suited for fabrication by integrated circuit techniques.
It is a further object of this invention to provide a digital timer which provides a choice of output time delays without requiring complicated decoding circuitry.